This invention is in the field of optical instrumentation related to apparatus for measuring concave, planar and convex surfaces, and more particularly related to apparatus for measuring certain critical anatomical dimensions of the human eye. It is especially suited for measurements related to the sagittal depth or altitude of the corneal apex above certain points predetermined on the cornea or sclera (white) of the eye.
The portion of an eye which forms the central frontal exterior surface is called the cornea. An important application for instruments which measure the sagital characteristics of the cornea is the fitting of contact lenses. The radius of curvature must be properly analyzed in topographical terms for the proper fit of contact lenses.
The prior art instrumentation used to measure anatomical dimensions of the eye and particularly corneal curvature is principally based on the phenomena that the image size reflected from a convex surface is directly proportional to the radius of curvature of a convex object. Placido's disc exemplifies such an instrument. Placido's disc is a round, flat disc with alternating black and white rings. When light is reflected from the disc to the human cornea it gives the observer a general idea of the coreal curvature and its regularity. The larger the reflected image the greater the radius of curvature.
Since Placido's rudimentary disc, instrumentation has evolved to better plot the exterior configuration of a corneal image and then convert this form into a measurement, more or less approximating the radius of curvature. Examples of instruments of this type are the Photo-Electric Keratometer, the Keratometer and the Ophthalmometer. Such devices are disclosed in U.S. Pat. No. 3,545,846 to Wilms, U.S. Pat. No. 3,511,561 to Gambs, and U.S. Pat. No. 1,750,931 to Kellner.
The inherent disadvantage of these prior art instruments is that they measure the cornea as if it were a true spherical shape. Actually, the cornea is not spherical and may assume many different configurations. Secondly, the size of the area of the cornea measured by these prior art devices is small in comparison to the total surface area of the cornea. Third, the prior devices measure the radius of curvature of the cornea at its center where the radius of curvature is usually steeper than at the sides. The typical Keratometer or Ophthalmometer measures a central section of the cornea from about 2.5 to 3 mm in diameter, or about 5% of the total area of the cornea. The average cornea, however, is about 12 mm in diameter, and the average contact lens diameter can vary from 7 mm to 14 mm in diameter. Experience in using these prior art instruments has taught that the practitioner derives very limited information upon which to base his selection of the correct curvature for a contact lens, especially since 95% of the corneal curvature is not included in the measurement and may vary greatly from the narrow central measurement. Hence, the practitioner must rely upon his experience and trial and error to achieve a final satisfactory fit. Frequently, new lenses must be changed or re-ordered to provide a satisfactory fit for the patient. This is a costly procedure, an inconvenience for the patient and less than optimal.
The Photo-Electric Keratometer was developed to measure a larger area of the cornea. The data obtained from the instrument consists of a series of radii but similarly assumes that the cornea is configured from a series of small spherical sections. The data generated is then supplied on a computer printout to give a composite curve. The practitioner can then determine a likely curvature for the contact lenses to be fitted. Again, the selection process for the proper curvature depends upon the practitioner's experience and judgment.
The above procedures are used for fitting a "hard" contact lens; however, the proper selection of the radius of curvature of a "soft" contact lens involves even more approximation and judgment. This derives from the fact that the diameter of the "soft" contact lens is larger than the corneal diameter and, in fact, extends over the outer border of the cornea and onto the sclera (white portion of the eye around the cornea). The average radius of curvature of a cornea is approximately 7.94 mm., while the average radius of curvature of the sclera is approximately 12.25 mm. As a consequence, the practitioner must select a base curve (inside curve) for a soft contact lens of approximately 14 mm. diameter. Such a contact lens is designed to cover the entire cornea plus a small portion of the sclera. In the case of a Keratometer or Ophthalmometer, this would require a fitting based on a measurable 2 to 3 mm. section of the cornea (or 5% of the total area of the cornea) with no data whatever on the sclera; therefore, the probability of a poor fit is even greater with a "soft" lens than a "hard" lens. None of the prior art satisfactorily addresses the problem of measuring the characteristics of the sclera portion of the eye.